Ukraine's new President Petro Poroshenko vowed at his inauguration ceremony Saturday to maintain the unity of his country, as the east is embroiled in an uprising by pro-Russian rebels.

Speaking in parliament, Poroshenko promised the residents of the Donbass region, which is largely in rebel hands, that he would decentralise power and guarantee the free use of the Russian language.

Poroshenko said that there would be no compromise with Russia on his pro-European stance and the status of the Crimean peninsula.

"The Crimea has been and will remain Ukrainian," he said. "I put that clearly to the Russian leader in Normandy," he added, referring to his meeting with President Vladimir Putin at D-Day commemorations on Friday.